The invention relates to a process for the isomerization of n-paraffins into isoparaffins, normally with the aim of improving the octane number of certain petroleum fractions and more particularly those containing pentanes and n-hexanes, as well as pentanes and branched hexanes (C.sub.5 /C.sub.6 fractions).
The existing processes for the isomerization of C.sub.5 /C.sub.6 hydrocarbons using high activity chlorinated alumina-type platinum catalysts operate on a once through basis or with partial recycling after fractionation of the unconverted n-paraffins, or with a total recycling after passing onto liquid phase molecular sieve systems.
Once through operation, although simple, lacks effectiveness in increasing the octane number. To obtain high octane numbers, it is necessary to recycle low octane number constituents, after passing either into separating columns (e.g. a deisohexanizer) or onto molecular sieves in the liquid or vapor phase.
A known isomerization process using molecular sieves for vapor phase separation of unconverted n-paraffins integrates the adsorption stage using the molecular sieve with the reaction stage. This is the total isomerization process (TIP) e.g. described in U.S. Pat. No. 4,210,771. It combines the use of an isomerization reactor supplied by the mixture of the charge, a desorption effluent and hydrogen and the use of a separating section for the adsorption of the n-paraffins on the molecular sieve, desorption being carried out by hydrogen stripping. In such a process, the reaction system cannot consist of a high activity chlorinated alumina stage due to the risks of contamination by the hydrochloric acid of the integrated molecular sieves. Use is then made of a less high performance, zeolite-based catalytic system not using chlorine. This leads to a product having an octane number 1 to 2 points below that which would have been obtained with a chlorinated alumina-based catalyst. Thus, it is known in the art that the lower the isomerization temperature, the higher the conversion of n-paraffins into isoparaffins and in addition, the better the conversion of low octane number C.sub.6 isomers (methyl-pentanes) into higher octane number C.sub.6 isomers (dimethyl butanes).
It is also known that the platinum-impregnated chlorinated alumina-based catalyst makes it possible to carry out the isomerization reaction at a lower temperature than the more stable, unchlorinated zeolite-type catalysts.
It is therefore of particular interest to have a process able to combine a low temperature reaction system (in order to bring about the best once through octane number) and a recycling system for the low octane number constituents in the non-integrated or chlorine-resistant form.
Consideration can be given to conventional systems using separating columns (deisopentanizer and deisohexanizer), because the separating columns can be immunized against contamination by chlorine. However, they require a large amount of equipment and have a high energy consumption and are consequently expensive to use. A system using a single separating column (deisohexanizer only) would be less expensive, but would not be able to convert all the n-pentane into isopentane and could consequently not make it possible to obtain the octane number increase levels of systems using recycling.
To avoid contamination by chlorine of the molecular sieves used for the separation, consideration can be given to a non-integrated system using a stabilization stage for the isomerization effluent before supplying it to the adsorption stage. This has been proposed in the combined, so-called PENEX MOLEX process, in which the C.sub.5 /C.sub.6 hydrocarbons are isomerized in a chlorinated alumina catalytic reaction, followed by adsorption on a molecular sieve in the liquid phase of the n-paraffins from the bottom of the stabilizer and at the bottom temperature. Use of a molecular sieve in adsorption and desorption in the liquid phase is more difficult to carry out than in the vapor phase. Thus, the relationship of the quantities of normal paraffins adsorbed to the quantities of isoparaffins present in the mobile phase is significantly in the favor of the vapor phase.
Another obstacle to the use of high activity catalytic systems is their sensitivity to the contaminants of the charge, namely sulphur and water. The liquid charge and the make-up hydrogen must be sweetened, freed from sulphur and dehydrated prior to introduction into the reaction system.
In the present state of the art using chlorinated alumina-based catalytic systems, the charges are dried in pretreatment operations by using several molecular sieves.